1. Technical Field
This invention relates to electrospray ion sources, and more particularly to capillaries useful for the transfer of ions generated by such sources.
2. Background Information
Electrospray ionization has become a common method of generating ions of sample material to be analyzed by mass spectrometers. An electrospray ion source, capillary assembly and mass spectrometer are shown in FIG. 1. In a spray chamber 2, substantially at atmospheric pressure, sample material is made to flow through a grounded conductive needle 4. A cylindrical wire screen electrode 6 typically surrounds the bulk of the chamber and is maintained at an appropriate potential. For example, a potential of -2500 volts is usual for electrospray of positive ions. Sample liquid emerges from the needle in a spray which forms the shape of a so-called "Taylor cone" 7 which breaks up into small charged liquid droplets at the tip. Evaporation yields smaller droplets and eventually sample ions. This stream of droplets and sample ions is directed into a downstream vacuum chamber 8 with a mass spectrometer 10 at its downstream end. The droplets and sample ions are guided to the capillary via an annular end electrode 12 at the downstream end of the spray chamber. In the case of positive ions the end electrode 12 would be at an even greater negative potential than the cylindrical electrode 6, for example at -3700 volts. The ions may then flow through the capillary under the influence of the pressure difference across the capillary.
A particular prior art capillary 20 is shown in greater detail in FIG. 2. The capillary comprises a unitary glass tube 21 having a cylindrical outer surface 23 and internal bore 22. With reference to its upstream to. downstream central longitudinal axis, the capillary is both radially symmetrical and symmetrical about its center along the axis. Each end of the tube has a slight bevel which, however, leaves a major central portion of the end remaining perpendicular to the axis. Conductors 26 are formed at each end of the capillary by a platinum coating (shown with extremely exaggerated thickness) which encompasses the outer surface of the tube 21 adjacent the tube's end as well as covering the end itself, excluding a central aperture 27 at the internal bore 23. Exemplary capillary dimensions are 7.125 inches long by 0.25 inches outer diameter by 0.025 inches internal diameter.
Electrical contacts (not shown) engage the capillary conductors to maintain the conductors at desired voltages to establish the space potential at the upstream and downstream ends of the bore. The upstream electrode of the capillary is maintained at an extreme negative potential, such as -4500 volts, than is the end electrode 12. The downstream conductor of the capillary may form the first stage of a multistage lensing system 14 for the final direction of the ions to the spectrometer. Typically, such lensing systems involve smaller absolute potentials than in the spray chamber, for example placing the downstream capillary conductor at a potential of 160 volts with subsequent lenses decrementing to -80 volts. In addition to the multiple voltage stages, multiple pressure stages may be provided to bring the pressure down to the ultimate pressure at the spectrometer of, for example, 10.sup.-5 torr. The voltages are typically reversed when using negative ions.
The foregoing discussion is, of necessity, a brief discussion of the functioning of electrospray ion sources and associated spectrometers. A number of ion sources are commercially available which would be suited for use with capillaries of the present invention. Exemplary units include those produced by Analytica of Branford, Inc., Branford, Conn., and supplied with spectrometers of Hewlett Packard.
Often, the interior surface of the capillary acquires deposits over time--typically of less volatile samples such as peptides or salts. Often the gold or platinum coating forming the conductors becomes damaged by electric discharges or field-induced oxidation, particularly in configurations wherein the electrospray needle is located particularly close to the capillary. This configuration is useful for microelectrospray or nanoelectrospray sources wherein particularly small volumes of sample material are involved.
A changeout of the capillary may be necessary, to replace it with a cleaner capillary or a different type of capillary, such as one having a different internal diameter or one having a desired conductivity. For changeout, the source may be turned off to stop the flow of ions through the capillary. The spray chamber may be opened to provide access to the capillary which is extracted from its retainer. The extraction of the capillary from its retainer permits an inflow of air into the low pressure chamber, breaking the vacuum. After changeout, it may take significant time, sometimes on the order of days, to reestablish the desired vacuum.